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United States Patent |
5,540,751
|
Yamamoto
,   et al.
|
July 30, 1996
|
Method for recovering zinc from zinc containing dust
Abstract
A method for recovering zinc from zinc comprising the steps of:
producing agglomerates containing carbon from dust which contains zinc in a
form of zinc oxides;
charging the agglomerates into molten metal, the zinc oxides in the
agglomerates being reduced and vaporized into a vaporized zinc; and
collecting the vaporized zinc as zinc oxide with a generated dust.
Inventors:
|
Yamamoto; Naoki (Kawasaki, JP);
Takemoto; Katsuhiro (Kawasaki, JP);
Sakamoto; Noboru (Kawasaki, JP);
Iwata; Yoshito (Kawasaki, JP)
|
Assignee:
|
NKK Corporation (Tokyo, JP)
|
Appl. No.:
|
349490 |
Filed:
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December 2, 1994 |
Foreign Application Priority Data
Current U.S. Class: |
75/376; 75/10.14; 75/658; 75/961 |
Intern'l Class: |
C22B 019/04 |
Field of Search: |
75/10.3,10.31,10.32,658,961,376,10.14
|
References Cited
U.S. Patent Documents
2127632 | Aug., 1938 | Najarian | 75/324.
|
3770416 | Nov., 1973 | Goksel.
| |
4612041 | Sep., 1986 | Matsuoka et al. | 75/658.
|
4917723 | Apr., 1990 | Coyne, Jr.
| |
4940487 | Jul., 1990 | Lugscheider et al.
| |
5279643 | Jan., 1994 | Kaneko et al. | 75/961.
|
5364441 | Nov., 1994 | Worner | 75/961.
|
Foreign Patent Documents |
0150805 | Aug., 1985 | EP.
| |
0551992 | Jul., 1993 | EP.
| |
1076156 | Feb., 1960 | DE.
| |
1162389 | Feb., 1964 | DE.
| |
58-144437 | Aug., 1983 | JP.
| |
Other References
Patent Abstracts Of Japan, vol. 7, No. 263 (C-196), 24 Nov. 1983 of JP-A-58
144 437 (Sumitomo Kinzoku Kogyo), 27 Aug. 1983.
Patent Abstracts Of Japan, vol. 8, No. 222 (C-246) (1659), 9 Oct. 1984 of
JP-A-59 107 036 (Shin Nippon Seitetsu), 21 Jun. 1984.
|
Primary Examiner: Andrews; Melvyn
Attorney, Agent or Firm: Frishauf, Holtz, Goodman, Langer & Chick, P.C.
Claims
What is claimed is:
1. A method for recovering zinc from a dust containing zinc and other
elements, comprising:
(a) agglomerating dust which contains zinc and other elements, the zinc
being in a form of zinc oxides, to form intermediate agglomerates;
(b) coating the intermediate agglomerates with a fine material containing
carbon to form a coating layer on the intermediate agglomerates to produce
final agglomerates, the final agglomerates containing carbon in an amount
of 5 to 40 wt.%;
(c) charging the final agglomerates from step (b) into a hot metal, the
zinc oxides in the final agglomerates being reduced and vaporized into a
vaporized zinc; and
(d) collecting the vaporized zinc as zinc oxide in a zinc concentrated
dust.
2. The method of claim 1, wherein said dust which is agglomerated in said
step (a) further contains carbon.
3. The method of claim 2, wherein said dust is a fine dust from a blast
furnace.
4. The method of claim 1
further comprising
curing the intermediate agglomerates from step (a) in a steam atmosphere.
5. The method of claim 1, wherein the carbon content of the final
agglomerates is 10 to 30wt.%.
6. The method of claim 1, wherein the final agglomerates are produced by
the following steps:
agglomerating in step (a) the zinc containing dust to form the intermediate
agglomerates;
curing the intermediate agglomerates in a steam atmosphere;
coating the cured intermediate agglomerates with carbonaceous material to
form the final agglomerates; and
drying the coated final agglomerates.
7. The method of claim 1 wherein the dust which is agglomerated in said
step (a) is a fine dust from a converter.
8. The method of claim 1, wherein said hot metal is tapped from a blast
furnace and which flows on a runner of a casting yard of a blast furnace.
9. The method of claim 1, wherein said hot metal is introduced into a hot
metal ladle.
10. The method of claim 1, wherein said fine material is at least one
selected from the group consisting of fine coke, pulverized coal and fine
dust from a blast furnace.
11. The method of claim 1, wherein said hot metal is at a temperature of
1500.degree. C.
12. The method of claim 1, wherein the coating layer has a carbon content
of 0.2 to 40 wt.% based on the weight of the agglomerates and the
agglomerates have a carbon content of 5 to 40 wt.% based on the weight of
the agglomerates.
13. The method of claim 1, wherein said fine material contains metallic
iron.
14. The method of claim 13, wherein the final agglomerates contain 1 to 90
wt.% metallic iron.
15. The method of claim 14, wherein the final agglomerates contain 50 to 75
wt.% metallic iron.
16. The method of claim 1, wherein said step step (a) further comprises
adding a binder in an amount of 1 to 20 wt.% to the dust which is
agglomerated in said step (a).
17. The method of claim 16, wherein the binder is added in an amount of 5
to 15 wt.%.
18. The method of claim 16, wherein the binder is at least one selected
from the group consisting of cement, quick lime and bentonite.
19. The method of claim 1, wherein the final agglomerates have the
following size distribution:
1 mm or less:10 wt.% or less,
40 mm or more:20 wt.% or less, and
over 1 mm and under 40 mm: the rest.
20. The method of claim 1, wherein said step (d) comprises the steps of:
drawing the zinc concentrated dust in an exhaust gas by suction;
continuously analyzing a zinc concentration of the zinc concentrated dust
in the exhaust gas; and
separating by dampers the exhaust gas into two streams of gas according to
the zinc concentration.
21. The method of claim 1, wherein said intermediate agglomerates are
pellets which are produced by a pan pelletizer.
22. The method of claim 1, wherein said intermediate agglomerates are
briquettes which are produced by a briquetting machine.
Description
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a method for recovering zinc from dust,
and more particularly to a method for recovering zinc from dust containing
iron as a main component.
2. Description of the Related Arts
Since a dust recovered in steel works has a high iron content, the dust is
efficiently utilized as an iron source. However, some of the dust contains
zinc. When such zinc containing dust is charged into a blast furnace, the
normal operation of the blast furnace may be hindered. Accordingly, a dust
containing zinc at an excessive amount can not be used as a raw material
for the blast furnace.
For example, a blast furnace dust collected by a dust catcher of the blast
furnace contains approximately 0.1 to 3 wt.% of zinc coming from iron ore,
and a converter dust collected by a dust collector contains approximately
0.1 to 3 wt.% of zinc coming from galvanized steel sheets or the like
which were charged to the converter as a scrap. When these dusts are
charged to the blast furnace, the zinc components are reduced to metallic
zinc, and the metallic zinc is melted to vaporize because it has a low
boiling point. The melting point of metallic zinc is 420.degree. C. and
the boiling point is 920.degree. C.
The vapor of metallic zinc ascends in the blast furnace along with the
reducing gas, and it is cooled while ascending. A part of the metallic
zinc vapor is discharged to outside of the blast furnace with the gas
(reducing gas) of the blast furnace. However, the rest of the metallic
zinc vapor adheres to the furnace wall surface in a form of liquid or
solid before reaching the top of the furnace. If the metallic zinc adheres
to the furnace wall surface and if it grows on the wall surface, then the
gas permeability within the furnace is decreased and the operating
condition of the furnace becomes abnormal. To prevent the furnace
operation from such a bad state, currently the zinc content of the burden
being charging into the blast furnace is controlled not to exceed 0.2 kg
per ton of hot metal. Accordingly, the application of the dust containing
zinc is performed after removing zinc.
A conventional method for removing zinc from a zinc containing dust and for
recovering it is disclosed in unexamined Japanese patent publication No.
144437/1983. FIG. 3 illustrates the disclosed method. The reference number
40 denotes a hopper holding zinc containing dust, 44 denotes a mixer car
holding a high temperature hot metal 62, and 48 denotes a wet separator
having a water tank for dust collecting. The reference number 41 denotes
an oxygen blowing pipe for transporting dust, 42 denotes a dust transfer
pipe, 43 denotes a top blowen lance, 45 denotes a hood, 46 denotes a duct,
and 47 denotes a fan.
The dust in the dust hopper 40 is transferred by oxygen blown through the
oxygen blowing pipe 41, and is injected into the hot metal 62 in the mixer
car via the top blowen lance 43. The blown dust is heated by the hot metal
62 and is reduced by carbon in the hot metal. The iron oxide in the dust
melts into the hot metal, and the zinc oxide in the dust is vaporized and
is sucked along with the powdered dust in the mixer car 44. The vaporized
zinc is sent to the wet separator 48 and are then collected in water.
However, the above-described method has following problems. Since powdered
dust is charged into the hot metal 62, bubbles generated in the hot metal
contain dust. When the ascended bubbles break at the surface of the hot
metal, the dust in the bubbles is carried over with the sucked flue gas or
suspended at the surface of the hot metal. The suspended dust is difficult
to melt into the hot metal, and a part of it is carried over. As a result,
the wet separator 48 accepts the dust which has been blown into the hot
metal and the dust is left in a state of being neither reduced nor
vaporized. Consequently, the collected dust shows a very low zinc content
and very high iron content, and the recovery efficiency of iron to hot
metal becomes low.
Since the blown gas includes oxygen, fine particles of iron oxide which are
newly generated by the blown oxygen are also carried over, and these
particles are also collected by the wet separator 48. Accordingly, the
zinc content in the collected dust is further lowered, and the zinc
content may become lower than that in the supplied dust depending on the
operating condition. Furthermore, the generation of the new fine particles
of iron oxide degrades the recovery rate of iron into the hot metal.
When oxygen is included in the blown gas, the atmosphere in the container
holding the hot metal likely becomes an oxidizing one. Therefore, the dust
floating on the hot metal surface is difficult to be reduced. As a result,
the quantity of zinc being vaporized is decreased, and the recovery rate
of zinc is decreased. Also the iron oxide is likely discharged along with
slag, and the iron recovery rate into the hot metal is lowered.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for recovering
zinc from zinc containing dust wherein the zinc in the dust is efficiently
reduced to vaporize and the zinc and iron in the dust are recovered at a
high recovery rate.
To attain the above-mentioned object, the present invention provides a
method for recovering zinc from zinc containing dust, comprising the steps
of:
producing agglomerates containing carbon from dust which contains zinc in a
form of zinc oxides;
charging the agglomerates into hot metal, the zinc oxides in the
agglomerates being reduced and vaporized into a vaporized zinc; and
collecting the vaporized zinc as zinc oxide with a generated dust.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing which illustrates an example of the present
invention.
FIG. 2 is an elevational view, partly in cross-section which illustrates
another example of the present invention.
FIG. 3 is an elevational view, partly in cross-section which illustrates a
conventional method for recovering zinc from dust.
FIG. 4 is a schematic diagram which illustrates an apparatus for bench
scale test of the present invention.
FIG. 5 in a graph which shows the change of zinc vaporization rate with
time after charging the pellets according to the present invention.
FIG. 6 is a schematic diagram which shows a mass balance of zinc before and
after charging pellets according to the present invention.
FIG. 7 are graphs which show the relation among carbon content, zinc
content in recovered dust, zinc recovery rate, iron recovery rate, and
unit consumption of carbonaceous material according to the present
invention.
FIG. 8 are graphs which show the relation among carbon content in coating,
reaction initiation time, and reaction completion time.
FIG. 9 in a graph which shows the definition of reaction initiation time
and reaction completion time.
FIG. 10 are graphs which shows the relation of added binder, zinc content
in recovered dust, zinc recovery rate, and iron recovery rate according to
the present invention.
FIG. 11 are graphs which show the relation among metallic iron content,
reaction initiation time, and reaction completion time according to the
present invention.
FIG. 12 is a graph which shows a change of zinc content in the recovered
dust with time after charging agglomerate according to the present
invention.
FIG. 13 is a graph which shows the relation among zinc content during dust
recovery and average zinc content of recovered dust; and
FIG. 14 is a graph which shows the relation between the number of recycles
of recovered dust and the zinc content in recovered dust according to the
present invention.
DESCRIPTION OF EMBODIMENT
In the present invention, agglomerates including carbon are produced from
zinc containing dust. The agglomerates arc charged into a hot metal. After
charging, the agglomerates are contacted with the hot metal. The
agglomerates are heated up by the hot metal and the carbon in the
agglomerates burns to make CO gas. The CO gas burns to become CO.sub.2 gas
on the surface of an agglomerate layer. This causes the agglomerates to be
heated more and more. Zinc oxide in the agglomerates is reduced by the CO
gas and vaporized. The vaporized zinc is changed to zinc oxide due to the
contact with air. The generated zinc oxide dust is collected.
When the dust contains sufficient amount of carbon for use as carbon
source, the agglomerates can be made from the zinc containing dust without
addition of carbonaceous material. When the dust does not contain
sufficient amount of carbon for use as carbon source, or when the dust
does not contain sufficient carbon for securing desired amount of carbon,
the zinc containing dust is agglomerated with a fine carbonaceous
material, and the agglomerate is charged onto the hot metal.
According to the present invention, the zinc containing dust is
agglomerated before charging it to the hot metal. Consequently, the
phenomenon in which the charged dust is carried over an as-charged state
hardly occurs, and dust which contains zinc at a high content is
recovered. The recovery of iron into the hot metal is also improved.
Also according to the present invention, carbon is added to the
agglomerate. The source of the carbon which is added to the agglomerate
differs with the type of zinc containing dust to be treated. For example,
when the zinc containing dust is a blast furnace dust containing a large
amount or carbon, the required amount of carbon is supplied from the blast
furnace dust, and no extra source of carbon is necessary to add. In
another example, there may be a need for the addition of fine coke or
pulverized coal to adjust the wanted level of carbon content. If the zinc
containing dust is a converter dust which contains a very low
concentration of carbon, the agglomeration is carried out by adjusting the
carbon content of agglomerate by adding carbonaceous material such as fine
coke and pulverized coal or blast furnace dust. The converter dust is
generated in the steel making process.
Since the hot metal is at a high temperature, the presence of carbon in the
agglomerate accelerates the reduction of agglomerate.
When the agglomerate is charged into the hot metal, the agglomerate floats
on the hot metal because specific gravity of the agglomerate is lower than
that of hot metal. Then, the carbon in the agglomerate contacts air to
burn, and generates carbon monoxide, which then brings the surrounding
atmosphere of the agglomerate to the reducing atmosphere. Furthermore, the
combustion of carbon occurs also in a micro-space inside of the grain of
the agglomerate, so the combustion makes the inside space of grains a
reducing atmosphere, also. The combustion of carbon heats the agglomerate,
and temperature raise of the agglomerate swiftly occurs. In this manner,
the carbon presence in the agglomerate brings the grains of agglomerate in
a reducing atmosphere, and the temperature rising speed increases, so the
reducing reaction of the zinc component and the iron component is
significantly accelerated.
A preferable carbon content in the agglomerates is in a range of from 5 to
40%. When the carbon content becomes less than 5%, the recovery rate of
zinc suddenly reduces. When the carbon content becomes to above 40%, the
rate of zinc recovery saturates and unit consumption of the carbonaceous
material to the recovered amount of zinc suddenly increases. A carbon
content of 20 to 40% is more preferable. The carbon content can also be 10
to 30 wt.%.
When a carbonaceous material is included in a coating layer on the
agglomerate, the combustion of the carbonaceous material promptly begins,
which then accelerates the temperature increase of the agglomerate.
Accordingly, the waiting time until the reducing reaction begins is
shortened. In this case, a slight amount of coating of carbonaceous
material induces the effect of shortening the waiting time. Even when the
coating layer includes the carbon in an amount of 0.2% to the total amount
of agglomerate, the shortening of the waiting time becomes significant.
However, when the carbon in the coating layer exceeds 40% to the total
amount of agglomerate, the rate of shortening the waiting time becomes
less and the unit consumption of carbon material increases, so an
excessive addition of carbon material is not preferable.
As described above, the inclusion of carbon material in the agglomerate
accelerates the heating of the agglomerate. There is another means to
accelerate the heating, in which a metallic iron having a high thermal
conductivity is added to the zinc containing dust during agglomerating
thereof. Mixed metallic iron in the agglomerate increases the speed of
heating grains of agglomerate and shortens the waiting time until the
reducing reaction begins. According to an experiment to determine the
amount of the metallic iron necessary to rapidly heat the grains of the
agglomerate into their inside portion, a preferred content of the metallic
iron is in a range of from 1 to 90%. When the content of the metallic iron
is less than 1%, the waiting time for starting the reducing reaction is
extended and the necessary period for the reducing reaction is also
extended. Therefore, the content of the metallic iron less than 1% is not
favorable. Even when the content of the metallic iron exceeds 90%, the
shortening rate of the waiting time becomes less, so the addition of the
metallic iron exceeding 90% is not necessary. The metallic iron content of
50 to 75% is more preferable. The metallic iron can be replaced by iron
oxide, which gives a similar effect.
As described above, the invention prevents the carry-over of the zinc
containing dust by agglomerating the powder zinc containing dust. However,
if the strength of grains of the agglomerate is poor, the grains are
broken when they are charged onto the hot metal owing to sudden
vaporization of water in the agglomerate, and a part of these grains
generate cracks and are powdered. The powdered agglomerates are carried
over to degrade the zinc content in the recovered dust and to degrade the
melting of dust into the hot metal bath, which then degrades the iron
recovery to the hot metal.
Responding to the phenomenon, this invention increases the strength of the
grains of the agglomerate by agglomerating the zinc containing dust with
the addition of binder such as cement to a degree of 1 to 20%, and
preferably 5 to 15 wt.%, as needed. If about 1% of the binder is added,
the carry-over of the agglomerate caused by breaking and powdering is
suppressed, and the zinc content in the recovered dust increases. However,
the addition of the binder at above 20%induces mutual, strong, and dense
binding of fine grains of dust which from the agglomerate, and forms a
state where the internal space within the grains is closed. Consequently,
the gas permeation through the internal space of the grain of the
agglomerate becomes difficult, and the necessary period for reducing
reaction extended. As a result, the specified reaction time passes without
completing the reaction, and the amount of discharge of unreacted
agglomerate increases, and the recovery rate of zinc and iron decreases.
A preferred agglomerate has a size distribution containing 10% or less for
1 mm or less of size and 20% or less for 40 mm or more of size. The lower
limit of the size distribution (less than 10% for 1 mm or less of size) is
determined to prevent carry-over of charged agglomerate. If the content of
grains of 1 mm or less exceeds the lower limit, the rate of carry-over of
the charged agglomerate rapidly increases. If the size distribution
exceeds the upper limit (20% or less for 40 mm or above) and if the
portion _ of coarse grains increases, then the heating time necessary to
raise the temperature of the agglomerate charged onto the hot metal
extends, and the rate of reducing reaction decreases. As a result, the
zinc recovery rate rapidly decreases.
Carry-over of zinc by vaporization begins after charging the agglomerate
into the hot metal, and after a certain time for zinc to be heated,
reduced, and melted has passed, then by vaporization, and ends at the
completion of the reducing reaction. After starting the reaction, the
quantity of vaporized zinc suddenly increases at a certain point, and it
rapidly decreases when the reaction approaches its end point.
Consequently, if the dust collection is conducted for recovering zinc
during a period for vaporizing a large amount of zinc, the dust containing
large amount of zinc is recovered. In this respect, a continuous analysis
of the zinc content in a generated dust along with the flue gas identifies
an inflection point where the zinc content shows a sudden increase and
decrease. Accordingly, if the desired value of the zinc content is set in
advance, if the dust collection is conducted to recover zinc when the zinc
content exceeds the set value, and if the dust collection is completed to
recover zinc when the zinc content becomes below the set value to
perform-the classified collection, then only dust containing a high
concentration of zinc is recovered at a high efficiency.
EXAMPLE
FIG. 1 illustrates an example of the present invention. The reference
symbol A denotes the process for agglomerating zinc containing dust. The
reference symbol B denotes the process for recovering zinc.
In the agglomeration process A, the reference number 30 denotes the raw
material hoppers which separately hold the blast furnace dust and the
converter dust which are the recovered materials containing zinc, and each
raw material is for agglomeration. The reference number 31 denotes the
mixer, 33 denotes the curing equipment, and 34 denotes the drier. The
reference number 35 denotes the carbonaceous material hopper for coating,
and 36 denotes the pelletizer for coating. The reference number 61 denotes
the agglomerate of such as the blast furnace dust and the converter dust
which are the recovered materials containing zinc.
In the zinc recovery process B, the reference number 1 denotes the blast
furnace, 2 denotes the runner, 3 denotes the tilting runner, 4 denotes the
hot metal ladle receiving hot metal 62. The reference number 10 denotes
the agglomerate charge hopper. The reference number 11 denotes the dust
collection hood, 12 denotes the flue gas duct, 13 denotes the continuous
analyzer, 14 denotes the dust collector for zinc recovery, 15 denotes the
general use dust collector, 16 denotes the blower, 17a and 17b denote the
flue gas switching dampers. The reference number 18 denotes the conveyer,
19a, 19b and 19c denote the recovered dust hoppers for recycling dust, 20
denotes the recovered dust hopper, 21a, 2lb, 21c, denote the recovered
dust hopper switching dampers for discharging dust, and 22 denotes
recovered dust switching dampers, and 23 denotes the general use dust
hopper.
The agglomerate is prepared in the following procedure. The procedure is
described in accordance with the agglomerating process A. The dust
containing zinc, such as blast furnace dust and converter dust and the
carbonaceous materials such as fine coke and pulverized coal, which are
held in the raw material hoppers 30, are discharged at a specified flow
rate and are introduced to the mixer 31. A binder such as cement, quick
lime, and bentonite, and a heat transfer assistant such as converter
coarse dust are charged to the mixer 31 at the same time, at need. The
mixture of dust and other components is fed to an agglomerating equipment
32 such as a pan type pelletizer and a briquetting machine. In the
agglomerating equipment 32, the mixture is granulated to an adequate size.
The granulated product is sieved to have a specified size distribution.
The sized granules are cured in the curing equipment 33 which are held in a
steam atmosphere at 120.degree. C. or more, then are dried in the drier 30
at 150.degree. C. or more to become the agglomerate 61. Before the sized
granules are cured and dried, they are coated with carbonaceous material,
at need. In that case, the granules arc fed to the pelletizer 36 for
coating such as a pan pelletizer or a drum pelletizer, and the fine coke
is taken out from the carbonaceous material hopper for coating and is
charged to the pelletizer 36 for coating. A specified amount of water is
further added to the pelletizer 36 for coating 36, then the surface of the
agglomerate is coated with fine coke.
The zinc recovery from the agglomerate is carried out in the following
procedure. The procedure is described in accordance with the agglomerating
process B. A specified amount of the agglomerate 61 is charged from the
agglomerate charge hopper 10 onto the hot metal 62 received by the hot
metal ladle 4. Since the specific gravity of the agglomerate 61 is less
than that of the hot metal, the agglomerate floats on the surface of pig
iron. The agglomerate is heated by the hot metal having a temperature of
about 1500.degree. C., and the carbon in the agglomerate burns. Then, the
zinc oxide and iron oxide in the agglomerate are reduced by carbon
monoxide generated by the combustion, and they become to metallic zinc and
metallic iron. The metallic zinc generated by the reducing reaction is
melted and is vaporized, and is converted to zinc oxide by contact with
air and the zinc oxide exists in the generated dust. The generated dust
containing zinc oxide is drawn with suction along with the flue gas, which
flue gas is then passes through the flue gas duct 12 and enters the dust
collector to be collected.
On the other hand, the metallic iron reduces its melting point while
absorbing carbon, and it melts down at 1150.degree. C. to be recovered in
hot metal.
Regarding the collection of the generated dust, the continuous analyzer 13
determines continuously the zinc content of the generated dust contained
in the flue gas flowing through the flue gas duct 12. If the zinc content
is in the above specified value, the flue gas switching damper 17a opens,
and the dust is collected at the dust collector for zinc recovery 14. If
the zinc content is below the specified value, the flue gas switching
damper 17b opens, and the dust is collected at the general use dust
collector 15, which collected dust is sent to the sintering plant as a raw
material for sintering.
The dust collected at the dust collector for zinc recovery 14 is
transferred by the conveyer 18 and is stored in the recovered dust
hoppers. Since the dust collected in the dust collector for zinc recovery
14 contains iron oxide, the zinc content is low, and the level of zinc
content is insufficient for effective use of the dust as a zinc source
without processing. Accordingly, the collected dust is again agglomerated,
vaporized, and recycled to raise the zinc content of the generated dust.
For example, in the case that the dust generated at the first vaporization
of the agglomerate prepared from the blast furnace dust and the converter
dust is collected, the recovered dust switching damper 21a opens, and the
dust transferred by the conveyer 18 is stored in the recovered dust hopper
for recycling dust 19a. The dust is then fed to the mixer 31 of the
agglomerating process A, where the dust is again agglomerated. The dust
generated at the vaporization of the secondary agglomerated product is
stored in the recovered dust hopper for recycling dust 19b. After
repeating these recycling treatments, the zinc content of the dust attain
a specified level. Then, the collected dust is stored in the recovered
dust hopper for discharging dust 20, and it is supplied as the zinc
source,
FIG. 2 illustrates another example of the present invention. The reference
numbers in FIG. 2 corresponding to the same components with FIG. 1 have
the same numbers one another, and the description is not given here.
According to the example, the agglomerate 61 is charged onto the hot metal
flow after eliminating the slag by the skimmer 5 discharged from the blast
furnace, or onto the hot metal flowing through the runner 2 at an elevated
temperature. The agglomerate charged into the hot metal ladle 4 floats on
the hot metal owing to the difference of specific gravity.
Then, the reaction which occurs is similar to the reaction which occurs
when the agglomerate is charged onto the hot metal which was filled in the
hot metal ladle 4 in advance. In this manner, when the agglomerate 61 is
charged onto the hot metal flow at an elevated temperature before being
charged into the hot metal 4, the agglomerate 61 is heated before it
reaches the hot metal ladle 4, and the heating period of the agglomerate
within the hot metal ladle 4 is shortened. As a result, the vaporization
of zinc swiftly begins.
The following are the test results. Table 1 lists the weight percentage of
the dust raw material components which structure the raw materials of
agglomerate.
TABLE 1
______________________________________
Total Metallic
Items Fe Fe Zn C SiO.sub.2
CaO
______________________________________
Blast furnace
35 1 1.94 32.3 6.4 3.2
Dust
Converter 67 15 2.34 1.7 1.5 2.2
Dust
Converter 85 72 0.03 0.8 1.2 4.0
Coarse Dust
______________________________________
EXAMPLE-1
A test was conducted using a test apparatus illustrated in FIG. 4. The
reference number 50 denotes the induction furnace, 11 denotes the dust
collection hood, 12 denotes the flue gas duct, 13 denotes the continuous
analyzer, 61 denotes the agglomerate, and 62 denotes the hot metal.
The blast furnace dust (zinc content of 2.34%) shown in Table 1 was mixed
with Portland cement and fine coke to agglomerate to obtain what is called
the cold bond pellets (agglomerate). The mixed amount of the cement and
the fine coke was 10% for each of them to the total amount of mixture. The
obtained pellets were sieved to size ranging from 3 to 10 mm.
The prepared pellets were charged onto the hot metal which was maintained
at a temperature of 1500.degree. C. at a rate of 20 kg/t-hot metal. The
state of reduction and melting of the pellets was observed.
A continuous analyzer was used to measure the zinc content in the generated
dust included in the flue gas sampled by suction to determine the change
of vaporized amount of zinc with time. Also the mass analysis of zinc
component was carried out by sampling specimens from dust in flue gas, the
hot metal after the completed reaction, and the slag. From these obtained
data, the mass balance of zinc before and after the charge of pellets was
prepared.
FIG. 5 shows the change of amount of zinc vaporized after the charge of
pellets with time. At about 3 min. after charging the pellets, the
reduction and vaporization of zinc began. The rate of vaporization reached
the maximum at about 7 min. Then, the vaporization was completed at about
12 min.
FIG. 6 shows a mass balance of zinc before and after the charge of pellets.
About 90% of zinc in the charged pellets was found in the generated dust,
about 8% was found in the hot metal, and about was found in the slag.
Consequently, the zinc recovery rate of the charged pellets was 90%, and
the iron recovery rate was 90%.
COMPARATIVE EXAMPLE-1
Converter dust having the composition as in Example 1 was charged in a form
of powder without agglomeration. The apparatus that was used was the same
as in Example 1. The zinc recovery rate of the charged converter dust was
70%. The iron recovery rate was 20%, which was far below that in Example
1. Furthermore, 50% of the charged converter dust was carried over into
the generated dust, so the zinc content in the generated dust was as low
as 0.55%.
EXAMPLE-2
Agglomerates having various grain sizes, which were prepared by the
compositions listed in Table 2 were tested by charging such agglomerates
onto the hot metal flow at the exit of a skimmer of a 4000 m.sup.3 blast
furnace during discharging, separately. The temperature of the hot metal
at the charging point was about 1500.degree. C., and the charge rate of
the agglomerate was 20 kg per ton of hot metal. The test results were
summarized in Table 2.
As seen in Table 2, when the size of the charged agglomerate was 100% for 1
mm of less, (the test level A1), the zinc content in the generated dust
was less than 1%, though the zinc recovery rate was extremely high. The
low zinc content was caused by carrying over a large amount of charged
material and by mixing the charged material which was not reduced nor
vaporized into the generated dust. When an agglomerate having a size of 40
mm or more was included at 25% or more, (test level AS), the zinc recovery
rate rapidly decreases. Consequently, it was found that a preferable size
distribution of the agglomerate was about 10% for 1 mm or less, and about
20% or less for 40 mm or more.
TABLE 2
__________________________________________________________________________
Agglomerate Results (%) Blending Ratio of Raw Material
Size (%) Zn Fe Zn Blast
Test
1 mm or
4 mm or
Recovery
Recovery
Content
Furnace
Fine
Pulverized Agglomerate
Level
less more Rate Rate in Dust
Dust Coke
Coal Cement
Shape
__________________________________________________________________________
A 1 100 0 95 10 0.95 100 15 -- 10 Pellet
A 2 10 0 90 60 18.32
100 15 -- 10 Pellet
A 3 5 10 88 60 20.25
100 -- 15 10 Pellet
A 4 1 20 86 50 25.28
100 15 -- 10 Pellet
A 5 0 25 60 30 30.32
100 15 -- 10 Pellet
B 1 5 10 89 62 19.48
100 15 -- 10 Briquette
__________________________________________________________________________
EXAMPLE-3
As shown in Table 3, agglomerates with various levels of carbon content
were prepared by mixing fine coke for adjusting the carbon content to a
converter dust and by mixing fine ore for adjusting the carbon content to
a blast furnace dust. These agglomerates were tested by charging such
agglomerate as onto the hot metal flow at the exit of a skimmer. The
results are summarized in FIG. 7. The unit consumption of the carbonaceous
material is represented by the formula of "carbonaceous material/recovered
Zn amount".
In FIG. 7, the symbol of a closed circle indicates the case that fine coke
was mixed into a converter dust, and the symbol of an open circle
indicates the case that fine ore was mixed into a blast furnace dust.
According to FIG. 7, when the carbon content becomes to 10% or more, the
zinc content in collected dust, zinc recovery rate, and iron recovery rate
rapidly increase. However, the carbon content of 40% or more is not
preferable because the unit consumption of carbon rapidly increases.
TABLE 3
__________________________________________________________________________
Mixing Ratio of Raw Material
Blast Additive for Charging
Test
Furnace
Converter
Adjusting Agglomerate
Point into
Level
Dust Dust Carbon Cement
Shape Hot Metal
__________________________________________________________________________
F -- 100 Fine Coke
10 Pellet Skimmer
Exit
G 100 -- Fine Iron
10 Pellet Skimmer
Ore Exit
__________________________________________________________________________
EXAMPLE-4
An intermediate agglomerate was prepared with the composition given in
Table 4. Then the intermediate agglomerate was coated by fine coke at
different amounts, separately. In that case, the total amount of the fine
coke within the grains or the agglomerate and the fine coke coated on the
grains was kept at 40% to the total amount of the agglomerate. These
agglomerates were tested by charging onto the hot metal flow at the exit
of a skimmer. The results are summarized in FIG. 8.
FIG. 8 shows the relation between the content of coated carbon, the
reaction initiation time (waiting time for beginning the reducing
reaction), and the reaction completion time. As seen in the figure, a
coating of fine coke as small as 0.2% of carbon equivalent amount
shortened the reaction initiation time to about half that of a non-coated
agglomerate, (from about 6 min. to 3 min.), and the reaction completion
time was also shortened to about two thirds (from about 30 min. to 20
min.) However, even when fine coke was coated to 40% of carbon equivalent,
the rate of shortening the reaction time became very small.
As shown in FIG. 9, the term "reaction initiation time" is defined as "the
time required for the zinc content in the recovered dust increases to 2%".
The term "reaction completion time" is defined as "the time required for
the zinc content in the recovered dust to become less than 2% after the
reaction initiation".
EXAMPLE-5
As shown in Table 5, fine coke was mixed into a converter dust, and further
a binder such as Portland cement, quick lime, or bentonite was added to
the basic oxygen furnace dust to agglomerate it while varying the added
amount of binder for test. In that case, the prepared agglomerates were
charged onto the hot metal which was received from a blast furnace into a
hot metal ladle. The results are summarized in FIG. 10.
In FIG. 10, the symbol of a closed circle indicates the case that cement
was used as the binder, the symbol of an open circle indicates the case
that quick lime was used as the binder, and the symbol of an open triangle
indicates the case that bentonite was used as the binder. As seen in the
figure, any type of binder tested showed a drastic increase in zinc
content of collected dust, zinc recovery rate, and iron recovery rate if
only the addition is 1% or more. However, it is not favorable that the
binder addition is more than 20% because the zinc and iron recovery rate
decreases.
TABLE 4
__________________________________________________________________________
Blending Ratio of Raw Material
Carbonaceous
Test
Blast Additive for
Material Agglomerate
Charging Point
Level
Furnace Dust
Adjusting Carbon
for Coating
Cement
Shape into Hot Metal
__________________________________________________________________________
H 100 Fine Coke
Fine Coke
10 Pellet Skimmer
Exit
__________________________________________________________________________
TABLE 5
__________________________________________________________________________
Blending Ratio of Raw Material Charging
Test
Converter
Fine
Pulverized Agglomerate
Point into
Level
Dust Coke
Coal Binder
Shape Hot Metal
__________________________________________________________________________
C 100 15 -- Cement
Pellet Hot Metal
Ladle
D 100 15 -- Quick
Pellet Hot Metal
Lime Ladle
E 100 -- 15 Bentonite
Pellet Hot Metal
Ladle
__________________________________________________________________________
EXAMPLE-6
With the composition given in Table 6, an agglomerate containing a heat
transfer accelerator was prepared. A converter dust was used as the heat
transfer accelerator, and the content of the heat transfer accelerator was
varied to prepare agglomerates having different metallic iron content.
These agglomerates were charged onto the hot metal which was discharged
from a blast furnace and was poured into a hot metal ladle. The results
are summarized in FIG. 11.
FIG. 11 shows the relation between the content of metallic iron, the
reaction initiation time, and the reaction completion time. As seen in the
figure, the metallic iron as small as 1% shortened the reaction initiation
time to about two thirds that of non-addition of converter coarse dust
agglomerate, (from about 6 min. to 4 min.), and the reaction completion
time was also shortened to about two thirds (from about 30 min. to 20
min.) However, when the content of metallic iron exceeds 90%, the rate of
shortening the reaction time became very small.
TABLE 6
__________________________________________________________________________
Blending Ratio of Raw Material
Blast Heat Charging
Test
Furnac
Fine Transfer
Agglomerate
Point into
Level
Dust
Coke
Cement
Accelerator
Shape Hot Metal
__________________________________________________________________________
I 100 15 10 Converter
Briquette
Hot Metal
Coarse Dust Ladle
__________________________________________________________________________
EXAMPLE-7
An agglomerate was prepared by mixing 100 parts of blast furnace dust, 15
parts of fine coke, and 10 parts of cement. The prepared agglomerate was
charged onto the hot metal which was received in a hot metal ladle. The
continuous analysis of zinc content in the generated dust with the flue
gas gave the change of zinc content with time as shown in FIG. 12.
Responding to the trend, the zinc content in the generated dust was
classified to a specific range to recover. In concrete terms, the
generated dust was recovered immediately after the agglomerate was
charged, and the dust collection began at each point of zinc content of
1%, 2%, 3%, 6%, and 10% for zinc recovery and completed the collection
when the zinc content reduced to the level of initiation of the
collection. The results are summarized in FIG. 13.
FIG. 13 shows the relation between the zinc content at the initiation of
dust collection and the average zinc content in the recovered dust. As
shown in the figure, when the zinc content in the generated dust becomes
2% is taken as the initiation of recovery, the average zinc content in the
obtained recovered dust rapidly increases and the efficient zinc recovery
is performed.
EXAMPLE-8
An agglomerate was prepared by mixing 100 parts of blast furnace dust, 15
parts of powdered coke, and 10 parts of cement. The prepared agglomerate
was charged onto the hot metal at the exit of skimmer. The zinc content in
the recovered dust was 17%. The recovered dust was used instead of the
above-described blast furnace dust to agglomerate. The obtained
agglomerate was again charged onto the hot metal flow. The zinc content in
the recovered dust on second recycle was 59%. In this manner, the
recovered dust was successively recycled to concentrate zinc. The results
are summarized in FIG. 14.
As shown in the figure, two times of recycle of the recovered dust gave
about 60% of the zinc content of the recovered dust. When the recovered
dust was recycled for two times, the zinc content becomes 50% or more,
which level is applicable as the zinc source. Therefore, a simple
recirculation of vaporized generated dust yields a zinc source.
According to the invention, a dust containing zinc which is a recovered
material containing zinc is charged into hot metal, and the zinc oxide in
the dust containing zinc is reduced and then vaporized. The vaporized zinc
is collected to recover. In that case, the charged zinc containing dust is
agglomerated in advance, so the carry-over of the charged zinc containing
dust occurs very little. As a result, a high zinc-content dust is
recovered, and the recovery of zinc and iron is performed at a high
recovery rate.
Since carbon is included in the agglomerate, the carbon burns in the
vicinity and inside of the grains of agglomerate. Consequently, the
atmosphere around and inside or the grains becomes a reduced one, and it
is heated to significantly accelerate the reducing reaction.
When the carbon source is added to an agglomerate in a state of coating on
the grains of agglomerate, the heat of hot metal rapidly burns the carbon
source on the surface of grains. As a result, the temperature increase of
agglomerate is accelerated, and the time of reducing reaction is
shortened.
When the agglomerate is formed by adding a binder, the strength of the
grains of agglomerate increases, and the carry-over caused by breaking or
powdering of the grains is suppressed. Consequently, a high zinc content
dust is recovered, and the recovery of zinc and iron is performed at a
high recovery rate.
Furthermore, when the agglomerate contains metallic iron, the heat
conductivity of the grains of agglomerate increases, and the rate of
heating to the inside of the grains increases. As a result, the time of
reducing reaction is shortened.
During the collection and recovery of vaporized zinc, the generated zinc
containing dust is sucked along with flue gas, and the zinc content of the
generated dust is continuously determined. The inflection point where the
observed zinc content shows a sudden increase and decrease is identified,
and the generated dust at or above the inflection point is collected.
Then, only the dust containing a large amount of zinc is efficiently
collected, and the high zinc content dust is recovered.
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